By virtue of a significant loss he could collect a healthy $1,100 — the winner-take-all pot for an online diet competition he’s playing with 21 friends and family members from around the country. He’s currently in the lead in a WeightLossWars.com challenge, but needs to lose a total of 40 pounds in a matter of weeks.

So to take it off, he put his game face on.

“I know what it means to compete. I’ve been an athlete in the past,” said the 33-year-old San Jose garbage and recycling worker. “I’m doing this for the same reason that I get worked up when I play simple games of Monopoly. I care about winning, and when it comes to my health, I decided that I’d finally had enough. It was just time.”

Witherspoon is one of thousands of men and women involved in online “diet betting” — putting their money where their mouths are, and wagering they’ll lose a certain amount of weight in a certain amount of time. Such websites are heating up this time of year when well-intentioned resolutions kick in and many need a kick in the pants to complete them.

While folks have been competing for years on their own through pencil-and-paper weight-loss wagers with co-workers and friends, online sites step it up more than a few notches with interactive charts and devices for tracking one’s progress, keeping an eye on the competition, learning about healthful eating and getting supportive boosts from fellow dieters.

“But adding the money component — it’s the kind of thing that gets your blood going,” said Pete Maughan, marketing director for WeightLossWars, which has about 130,000 users currently engaged in private challenges between friends, or public challenges in which anyone can join in. The site launched in 2004, which was also the first year of “The Biggest Loser,” the TV show that made weight-loss contests popular. And now more and more sites have popped up.

Gain by losing

“People really do get competitive,” Maughan said. “They want to win the money and the bragging rights. At the same time, you never see someone that truly doesn’t want the others to lose weight too. They just want to lose a little bit more.”

While research varies on the effectiveness of financial incentives for dieting, one 2008 study published in the Journal of the American Medical Association showed those in a control group in which money was at stake lost more weight than those who did not put some cash on the line.

On most diet-betting sites, competitors set up their own games — stakes, time frames, prizes — then the sites facilitate them, collecting and distributing the money, often through credit cards or PayPal. Regular weigh-ins are usually required, either through an honor system or with help from a “referee” such as a designated co-worker, a doctor or a health club. Money is optional on some sites, but for the most part the games resemble your typical office football pool — the only money you risk is what you put into the pot.

DietBet.com is the latest site on the scene with the motto of “Lose weight with friends while taking their money.” It launched in December and already has hundreds of people signed up in various contests.

“Ours is not winner-take-all like ‘The Biggest Loser’ show,” said founder Jamie Rosen. “Anyone who reaches their goal at the end of a four-week challenge splits the pot. It doesn’t matter who crosses the finish line first. We’re not making this like a gambling pool, but actually trying to help people lose weight.”

Some sites allow contestants to donate money to a charity if they don’t make their goals. Or even better: Fork over the bucks to an “anti-charity.” On StickK.com, you set goals, any goals — it could be weight loss, stopping smoking, being a better boyfriend or girlfriend, or cleaning the mini-blinds by Sunday — then you set your rules, one of which can be that, if you don’t meet your goal, you donate money to an organization you dislike.

“We don’t take a stand on groups one way or another, but say you’re totally against the NRA — you can set that as your anti-charity,” said Sam Espinosa of StickK. “If you miss your goal, the money goes to them. So not only are you losing your money, but you’re giving it to something you hate.”

Strong motivation

Espinosa says the anti-charity technique has proved to be the strongest motivator on his site, eliciting the most success. It has certainly worked for Mac Zilber, 19, of Albany, a student at UC San Diego. He has used StickK challenges to stop cracking his knuckles and chewing his nails. And his New Year’s resolution for 2011 was to do at least 45 minutes of vigorous exercise three times a week all year along.

“Any time I didn’t meet my goal, I would have to give $5 to my anti-charity, which was the George W. Bush library,” Zilber said. “That was sufficient disincentive for me, and I was successful meeting my goals the vast majority of the year. Even though $5 is not much, I didn’t want to give it to George.”

Lindsay Seltzer, 34, of San Francisco, who works as a marketing/consumer promotions professional in the wine industry, has also found financial incentives helpful. After struggling with her weight all her life — even undergoing gastric bypass surgery a few years ago, but eventually gaining the weight back — she recently achieved what she called a “mental shift” and lost 40 pounds in 2011.

“Seeing success brought out my competitive nature,” she said, and in November she joined two challenges through WeightLossWars with a cumulative pot of about $150, and another managed by a friend, worth about $160. Getting to 175 pounds before July is her current goal.

“Finally having a sense of confidence that it was something I could do and be successful with — that made me want to compete,” she said. “Really, the money is a bonus.”

Websites

StickK.com: Allows you to create personal “commitment contracts” to achieve a goal. Options are provided to set your own stakes, including giving money to an “anti-charity” — a group you dislike — if you miss your goal.WeightLossWars.com: Offers weight-loss or exercise competitions with your friends and family, or a public competition. Diet tracking tools are provided to monitor calories and exercise minutes.DietBet.com: Allows you to set up a free four-week game with friends to drop pounds. You decide how much everyone bets into the pot. Whoever loses the weight ﻿wins.